Patterns of the formation subsurface brackish water in the Nemunas River valley were investigated in the environs of Birštonas town (Middle Lithuania) as a typical case. A model comprising seven aquifers and the aquicludes confining them over an area of 80 km² was developed using MODFLOW and FEFLOW software. In order to justify its boundary and the initial conditions and to select the parameters, preliminary studies were performed including an analysis of the Pleistocene succession, the layout of palaeoincisions filled with deposits of different age and lithological composition as well as the investigations of carbon and oxygen isotopes (¹⁴C, ¹³C, ¹⁸O), tritium (³H) and helium (³He + ⁴He) in the groundwater.
The modeling data showed that the brackish water in the Birštonas area is formed by a brine discharge from deep aquifers in the zone of tectonic faults and palaeoincisions. The bulk discharge of the brackish water into the unconfined aquifer and the Nemunas channel takes place in a zone approximately 5 km long and 4 km wide, which almost coincides with an incision of the pre-Quaternary
surface. The average rate of the groundwater discharge per 1 km of the channel is about 850 m³/day.
The discharge of a solute (in terms of solids dissolved in groundwater) into the Nemunas River channel corresponding to an approximately twenty-kilometer-long river sector in Birštonas’ environs is about 36.5 t/day (1.8 t/day per 1 km of the channel).

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